Completed in 1136, The History of the Kings of Britain traces the story of the realm from its supposed foundation by Brutus to the coming of the Saxons some two thousand years later. Vividly portraying legendary and semi-legendary figures such as Lear, Cymbeline, Merlin the magician and the most famous of all British heroes, King Arthur, it is as much myth as it is history and its veracity was questioned by other medieval writers. But Geoffrey of Monmouth's powerful evocation of illustrious men and deeds captured the imagination of subsequent generations, and his influence can be traced through the works of Malory, Shakespeare, Dryden and Tennyson.
The other thing to review is this edition. Good translation, but awful apparatus. I really needed something to tell me what, if anything, was historically accurate and what was pure fantasy. As it is, I kind of sort of remembered some names from Bede or recent histories of dark ages Britain (Penda, for instance). I would have loved some footnotes giving me a bit more information; it also would have made the text itself more interesting.
In any case, well worth reading. I'm ready to move on to some later Arthuriana.